1. Field of the Invention
The present invention relates to an alloy coating for use as a surface coating which can prolong the service life of members for apparatuses for high temperature applications (hereinafter referred to as xe2x80x9cmembers for high temperature apparatusesxe2x80x9d), such as gas turbine blades, jet engines, and heat-transfer tubes for boilers, a method for forming the same, and a member for high temperature apparatuses.
2. Description of the Related Art
For members for high temperature apparatuses, such as industrial gas turbine blades and heat-transfer tubes for boilers, in many cases, a coating is applied on the surface thereof to improve the heat resistance and the corrosion resistance of the members. In general, in order to improve the heat resistance, a ceramic coating called xe2x80x9cthermal barrier coatingxe2x80x9d (hereinafter referred to as xe2x80x9cTBCxe2x80x9d) is applied to the surface of a substrate. Such a ceramic coating, however, has the problem that the difference in coefficient of thermal expansion between the substrate metal and the ceramic is so large that, when a ceramic layer is directly formed on the surface of the substrate, the ceramic layer is likely to be separated from the interface of the ceramic layer and the substrate. In order to solve this problem, in general, as shown in FIG. 3C, an undercoat 50 formed of an alloy layer and a topcoat 52 formed of, for example, a ceramic such as ZrO2 are laminated in that order on the surface of the substrate 10, to improve the adhesion of TBC 54 to the substrate 10.
Under very high-temperature environments of about 800 to 1200xc2x0 C., however, as shown in FIG. 3C, the undercoat 50 is reacted with the substrate 10 to form an Al-deficient layer 56 and a layer 58 of Al2O3+NiAl2O4, having poor protective properties at the interface of the undercoat 50 and the substrate 10, thus resulting in deterioration of TBC 54. Further, since TBC 54 is porous, atmosphere gas enters the inside of TBC 54 and then internally oxidizes or internally nitrides the substrate 10 to form an internal corrosion layer containing an internal oxide 36 and an internal nitride 38 within the substrate 10. For this reason, the service life of the member for high temperature apparatuses is as short as several months. This is a severe problem involved such members having a conventional ceramic coating. In a prior art technique, an attempt has been made to use a Ptxe2x80x94Al spray coating after Pt plating, usually, to improve the heat resistance. This technique, however, cannot attain a satisfactory improvement.
On the other hand, in order to improve the corrosion resistance, coating techniques such as diffusion coating of Cr or Al and spray coating of a high Ni-high Cr alloy have been utilized. Such coatings, however, have the drawback that when the coated member is used under a very high-temperature environment of about 800 to 1200xc2x0 C., the element contributing to the corrosion resistance is very rapidly diffused and, in addition, is highly reactive. Thus, the protective coating cannot be maintained stably for a long period of time. Further, even in the temperature range of 500 to 800xc2x0 C., when the member is used in a strongly corrosive environment containing, for example, chlorine (Cl) or sulfur (S), the element constituting the protective coating, such as Cr or Al, is rapidly consumed. Thus, again, the protective coating cannot be stably maintained for a long period of time, resulting in a short service life of the apparatus. For the above reasons, at the present time, the prolongation of the service life of a member for high temperature apparatuses is only made by lowering the service temperature of the apparatus while sacrificing the performance of the apparatus.
As described above, in the case of members used at high temperatures, even when TBC is applied to the members to improve the heat resistance, the TBC layer is deteriorated as a result of a reaction of TBC with the substrate during use of the members, and, in addition, the substrate is internally oxidized or internally nitrided by atmosphere gas which has penetrated into the inside of the coating. Further, even with the application of the diffusion coating of Cr or Al, the spray coating of a high Ni-high Cr alloy, etc., for improving the corrosion resistance, when the apparatus is used under an environment containing Cl, S or the like, which is highly corrosive at high temperatures, the consumption rate of the element constituting the protective coating, such as Cr or Al, is so high that a stable protective coating cannot be maintained for a long period of time.
It has now been found that rhenium (Re), iridium (Ir), niobium (Nb), tantalum (Ta), molybdenum (Mo), and tungsten (W), which are high-melting metals, when alloyed with an element for imparting corrosion resistance, such as chromium (Cr), aluminum (Al), silicon (Si), magnesium (Mg), niobium (Nb), tantalum (Ta), nickel (Ni), cobalt (Co), iron (Fe), molybdenum (Mo), iridium (Ir), tungsten (W), platinum (Pt), or rhodium (Rh), to form an alloy phase,.are stable at a high temperature of 1100xc2x0 C. or above, even at 1150xc2x0 C. or above, and, at the same time, possess excellent oxidation resistance. Further, as a result of studies on a high temperature corrosion reaction of a Ni-base superalloy coated with a Re (Ir, Nb, Ta, Mo, and W)xe2x80x94X alloy (wherein X=Cr, Al, Si, Mg, Nb, Ta, Ni, Co, Fe, Mo, Pt, Rh, Ir, W or the like), it has been found that an alloy phase containing Re (Ir, Nb, Ta, Mo, and W) can inhibit the outward diffusion of Ni, Al, Ti, Ta or the like and the inward diffusion of an oxidizing agent or the like. Thus, it is expected that provision of a thin layer of this alloy based on Re (Ir, Nb, Ta, Mo, and W) on the surface of a substrate can realize the inhibition of the outward diffusion of the alloying element from the substrate and the inward diffusion of the oxidizing agent or the like from the environment, which has been unsatisfactory with the conventional Ptxe2x80x94Al sprayed coating, leading to less corrosion and damage in the substrate whereby the service life of the member for high temperature apparatuses would be prolonged. Further, for members on which TBC is to be applied, when the above-described coating is interposed between the substrate and TBC, the interposed coating is considered to function as an excellent diffusion barrier which can inhibit the deterioration of TBC, caused by a reaction of TBC with the substrate, and the internal corrosion of the substrate.
As coating techniques used for improving heat resistance and corrosion resistance, PVD, CVD and spray coating are known. These methods have the drawbacks that: {circle around (1)} control of the thickness and composition of the coating is difficult; {circle around (2)} a large coating apparatus is needed, and the operation is complicated; {circle around (3)} the formed coating has many defects and cracks; {circle around (4)} there is a number of limitations on the size and shape of a substrate to be coated (for example, coating faithfully conforming to the shape of concaves and convexes is difficult); and {circle around (5)} the cost is high. In contrast, the technique of plating in a molten salt has the advantages that {circle around (1)} the thickness and composition of the coating can be easily controlled; {circle around (2)} the coating can be effected with ease by using a simple apparatus; {circle around (3)} a dense coating having no significant defect can be formed; {circle around (4)} there is few limitations on the size and shape of a substrate to be coated; and {circle around (5)} the cost is low.
The present invention has been accomplished under the above circumstances.
It is therefore an object of the present invention to provide an alloy coating which, when applied to members for high temperature apparatuses, can prolong the service life of the members, and a method for forming the alloy coating, and a member for high temperature apparatuses, to which the alloy coating has been applied.
The alloy coating according to the present invention comprises an alloy, the alloy comprising: at least one member, as a base, selected from the group consisting of Re, Ir, Nb, Ta, Mo, and W; and at least one alloying element for imparting corrosion resistance. The alloy coating is stable at a high temperature of 1100xc2x0 C. or above, even at 1150xc2x0 C. or above, and, at the same time, possesses excellent corrosion resistance and can inhibit the outward diffusion of Ni, Al or the like and the inward diffusion of an oxidizing agent or the like.
Alloying elements usable for imparting corrosion resistance include, for example, Cr, Al, Si, Mg, Nb, Ta, Ni, Co, Fe, Mo, Ir, W, Pt, and Rh.
A first embodiment of the method for forming an alloy coating according to the present invention comprises of conducting plating in a molten salt containing a chloride or a fluoride of at least one member selected from the group consisting of Re, Ir, Rh, Pt, Nb, Ta, Mo, and W, and a chloride or a fluoride of at least one alloying element for imparting corrosion resistance. This method can form an alloy coating while utilizing the above-mentioned advantages of the plating in a molten salt. Further, since a dense alloy coating having no significant defect can be formed, satisfactory properties can be imparted to the conventional Pt-base and Rh-base alloy coatings which have been unsatisfactory in heat resistance and corrosion resistance.
A second embodiment of the method for forming an alloy coating according to the present invention comprises: adding at least one member selected from the group consisting of Re, Ir, Rh, Pt, Nb, Ta, Mo, and W and at least one alloying element for imparting corrosion resistance, to a supporting salt of a chloride or a fluoride; introducing chlorine gas or hydrogen chloride gas into the mixture to produce a chloride of the elements in a molten salt; and conducting plating in the molten salt. According to the second embodiment of the method for forming an alloy coating, a dense alloy coating free from defects such as cracks can be formed at lower cost.
A third embodiment of the method for forming an alloy coating according to the present invention comprises of conducting plating in a molten salt containing a first chloride or fluoride of at least one member selected from the group consisting of Re, Ir, Rh, Pt, Nb, Ta, Mo, and W, and a second chloride or fluoride of at least one alloying element for imparting corrosion resistance, wherein an alloy of at least one metal contained in the first chloride or fluoride with at least one metal contained in the second chloride or fluoride is used as an electrode. According to the third embodiment of the method for forming an alloy coating, since plating can be effected in a stable manner, a denser alloy coating can be formed and, in addition, the composition of the alloy coating can be easily controlled.
A fourth embodiment of the method for forming an alloy coating according to the present invention comprises of conducting plating in a molten salt containing a first chloride or fluoride of at least one member selected from the group consisting of Re, Ir, Rh, Pt, Nb, Ta, Mo, and W, and a second chloride or fluoride of at least one alloying element for imparting corrosion resistance, wherein at least one metal contained in the first chloride or fluoride and at least one metal contained in the second chloride or fluoride are used as electrodes to carry out alternate or simultaneous plating. According to the fourth embodiment of the method for forming an alloy coating, the composition and structure of the alloy coating can be controlled as desired.
In the first through forth embodiments of the method for forming an alloy coating according to the present invention, described above, a chloride or a fluoride of alloying element for being base, such as Re, and a chloride or a fluoride of alloying element for imparting corrosion resistance, such as Al, are used at a mole ratio from Al:Re=1:0.01 to Al:Re=1:0.5, desirably a mole ratio from 1:0.02 to 1:0.2. Plating temperature is in the range of 150xc2x0 C. to 800xc2x0 C. Plating time is in the range from 10 minutes to 10 hours.
In the second embodiment of the method for forming an alloy coating according to the present invention, a chloride or a fluoride of an alkali metal or alkaline metal in the periodic table is used as a supporting salt of a chloride or a fluoride.
A first embodiment of the member for high temperature apparatuses according to the present invention comprises a substrate and an alloy coating covering the surface of the substrate, the alloy coating comprising an alloy, the alloy comprising: at least one member, as a base, selected from the group consisting of Re, Ir, Nb, Ta, Mo, and W; and at least one alloying element for imparting corrosion resistance. According to the first embodiment of the member for high temperature apparatuses, a deterioration in substrate caused by corrosion and damage can be reduced by virtue of the alloy coating covering the surface of the substrate, leading to a prolonged service life of the member.
A second embodiment of the member for high temperature apparatuses according to the present invention comprises an alloy coating interposed between a substrate and a thermal barrier coating, the alloy coating comprising an alloy, the alloy comprising: at least one member, as a base, selected from the group consisting of Re, Ir, Nb, Ta, Ho, and W; and at least one alloying element for imparting corrosion resistance. According to the second embodiment of the member for high temperature apparatuses, a deterioration in thermal barrier coating, caused by a reaction of the thermal barrier coating with the substrate, and the penetration of atmosphere gas into the substrate can be prevented by virtue of the alloy coating interposed between the substrate and the thermal barrier coating, leading to a prolonged service life of the member.
A third embodiment of the member for high temperature apparatuses according to the present invention comprises a substrate and an alloy coating, the member being prepared by plating a substrate in a molten salt containing a chloride or a fluoride of at least one member selected from the group consisting of Re, Ir, Rh, Pt, Nb, Ta, Mo, and W, and a chloride or a fluoride of at least one alloying element for imparting corrosion resistance, thereby forming an alloy coating on the surface of the substrate. According to the third embodiment of the member for high temperature apparatuses, a deterioration in substrate caused by corrosion and damage can be reduced by virtue of the alloy coating, which can function as a better diffusion barrier, covering the surface of the substrate, whereby the service life of the member is prolonged.
Here a thermal barrier coating may be formed on the surface of the alloy coating. In this case, a deterioration in thermal barrier coating, caused by the reaction of the thermal barrier coating with the substrate, and the penetration of atmosphere gas into the substrate can be prevented by virtue of the alloy coating, which can function as a better diffusion barrier, interposed between the substrate and the thermal barrier coating, whereby the service life of the member is prolonged.
A fourth embodiment of the member for high temperature apparatuses according to the present invention comprises a substrate and an alloy coating, the member prepared by a method comprising the steps of: adding at least one member selected from the group consisting of Re, Ir, Rh, Pt, Nb, Ta, Mo, and W and at least one alloying element for imparting corrosion resistance, to a supporting salt of a chloride or a fluoride; introducing chlorine gas or hydrogen chloride gas into the mixture to produce a chloride of the elements in a molten salt; and plating a substrate in the molten salt thereby forming an alloy coating on the surface of the substrate.
A fifth embodiment of the member for high temperature apparatuses according to the present invention comprises a substrate and an alloy coating, the member being prepared by plating a substrate in a molten salt containing a first chloride or fluoride of at least one member selected from the group consisting of Re, Ir, Rh, Pt, Nb, Ta, Mo, and W, and a second chloride or fluoride of at least one alloying element for imparting corrosion resistance, wherein an alloy of at least one metal contained in the first chloride or fluoride with at least one metal contained in the second chloride or fluoride is used as an electrode, thereby forming an alloy coating on the surface of the substrate.
A sixth embodiment of the member for high temperature apparatuses according to the present invention comprises a substrate and an alloy coating, the member being prepared by plating a substrate in a molten salt containing a first chloride or fluoride of at least one member selected from the group consisting of Re, Ir, Rh, Pt, Nb, Ta, Mo, and W, and a second chloride or fluoride of at least one alloying element for imparting corrosion resistance, wherein at least one metal contained in the first chloride or fluoride and at least one metal contained in the second chloride or fluoride are used as electrodes to conduct alternate or simultaneous plating of the substrate, thereby forming an alloy coating on the surface of the substrate.
The above and other objects, features, and advantages of the present invention will be apparent from the following description when taken in conjunction with the accompanying drawings which illustrates preferred embodiments of the present invention by way of example.